Ceramic whisker reinforced metal matrix composites offer many unique properties that make them attractive for structural use. Among these are high compressive strength, high fracture toughness and excellent creep characteristics.
Conventional practices for producing whisker reinforced metal-ceramic composites (metal matrix composites or cermets) involve blending of metal powders with separately produced whisker materials. Subsequently, the blend is de-gassed, compacted and hot pressed into a dense final state. The whisker materials are typically composed of silicon carbide (SiC), graphite, or refractory metal fibers. Examples of SiC whisker growth are demonstrated by Arco Silag, LANL vapor-liquid-solid (SiC), Nippon Carbon's Nicalon, and Sumitomo's SiC. Methods of graphite whisker growth are demonstrated by M. Endo's graphite whisker. Methods of refractory metal whisker production are demonstrated by Schladitz, U.S. Pat. No. 3,770,492.
Several difficulties are encountered in the production of conventional whisker reinforced composites. Externally produced whiskers are inevitably exposed to a certain amount of surface contamination during the whisker formation process and/or during subsequent handling steps. The surface contaminants, such as oxides, result in a deleterious layer or coating at the whisker-to-metal interface in the composite. Such layers inhibit interfacial bonding between the whiskers and the metal matrix, adversely effecting ductility of the composite. The weakened interfacial contact may also result in reduced strength, loss of elongation, and facilitated crack propagation. Additionally, the presence of surface contamination may inhibit wetting of the whiskers by molten matrix metal, rendering preparation of the composite difficult if not impossible. Consequently, many conventional whisker reinforced composites are not capable of being remelted, due to the tendency of the non-wetted whiskers to segregate from the molten matrix metal. Further, externally produced whiskers tend to agglomerate during powder blending steps, resulting in a non-uniform distribution of whiskers throughout the metal matrix. Also, at moderately high operating temperatures, conventional whiskers often deteriorate and/or react with the matrix metal. Another disadvantage is that refractory whisker materials have limited commercial availability, with associated high costs. In addition, handling of conventional whiskers in the 0.05 to 3.0 micron size range may potentially present hazards due to the pyrophoric nature as well as health hazards associated with the inhalation of very fine particles.
Molten metal infiltration of a continuous skeleton of second phase material has also been used to produce whisker reinforced composites. In this technique, preformed whisker material, such as silicon carbide, is pressed to form a compact, and liquid metal, such as aluminum, is forced into the packed bed to fill the intersticies. In the production of SiC/Al by this method, elaborate particle coating techniques have been developed to protect the SiC whiskers from the molten aluminum during infiltration and to improve bonding between the SiC and aluminum. Such a technique is illustrated in U.S. Pat. No. 4,444,603 to Yamatsuta et al, hereby incorporated by reference. In addition to the disadvantages associated with externally produced whiskers noted above, molten metal infiltration techniques further necessitate molten metal handling and the use of high pressure equipment.
The present invention overcomes the disadvantages of the prior art noted above. More particularly, the present invention provides for a cleaner whisker/metal interface compared with conventional whisker reinforced composites made by techniques using preformed whiskers because the reinforcing whiskers are formed in-situ. The clean whisker/metal interface achievable by the present invention results in highly improved mechanical properties. For example, the whisker reinforced composites of the present invention may exhibit fiber pullout under fracture, resulting in increased fracture toughness. A further advantage is that the whisker materials of the present invention exhibit excellent stability in metals at high temperatures, i.e. in excess of 1800.degree. C. Additionally, the complex ceramic whisker reinforced composites of the present invention can be produced at costs well below those of current whisker technologies, using simplified procedures and equipment compared to the prior art.
The production of metal matrix/ceramic composites using a solvent assisted in-situ precipitation technique is described in U.S. Pat. No. 4,710,348, issued Dec. 1, 1987 to Brupbacher et al, hereby incorporated by reference, and in the following U.S. patents: U.S. Pat. Nos. 4,774,052, issued Sept. 27, 1988, to Nagle et al; 4,751,048, issued June 14, 1988, to Christodoulou et al; and 4,772,452, issued Sept. 20, 1988, to Brupbacher et al; each of which is hereby incorporated by reference. These disclosures are primarily directed to the formation of metal matrix composites comprising particles of binary ceramic materials, such as TiB2, ZrB2 and TiC, dispersed throughout metallic matrices. These in-situ precipitated ceramic particles are typically of equiaxed shape, as opposed to the whisker shaped particles of the present invention. The formation of complex ceramic particulates is taught as is the preparation of particulates of varying morphology.
With these facts in mind, a detailed description of the invention follows which achieves advantages over known methods of producing whisker reinforced composites.